The present invention relates to electrical resistance sensors, and more particularly concerns sensors for monitoring plating processes. Monitoring of an electroplating process, as the process is carried out, is important to enable precise plating thicknesses and the high precision plating needed for many applications, such as use of precious metal platings. Precision measurement of plating thickness during electroplating has not been available.
Electrical resistance probes have been widely used for corrosion measurement, indicating by change of resistance the amount of metal that has been lost by corrosion or erosion over a period of time. A widely used sensor for this type of measurement is known under the trademark CORROSOMETER, manufactured by Rohrback Cosasco Systems, assignee of the present application. One such sensor employs a tubular metallic test element loop or wire, part of which operates as a reference element, made of the same material as the test element. The reference element is protected from the environment while the test element is exposed to the environment. A small alternating current is passed through the elements, and electrical resistance of each is measured while or after the sensor has been immersed in an environment in which a measurement is to be made. Because resistance varies with the amount of metal in the test element, measurement of test element resistance provides an indication of metal loss and therefore of corrosion.
Because resistance of the metal also changes with temperature, a reference element is provided, made of the same material as the test element and having the same temperature resistance characteristics. By this means changes in resistance of the test element that are due to long term relatively slow temperature variation may be eliminated by comparison of resistances of the test and reference elements. Other electrical resistance sensors may include a cylindrical metallic test element carrying an inner reference element made of the same material as the test element, with the interior of the test element filled with a thermally conductive electrically nonconductive compound, thereby providing physical support for the preferably very thin wall of the sensitive test element. It has been suggested to use such electrical resistance sensors to monitor plating operations. If used in a plating process, measurement of test element resistance would provide an indication of the plating thickness that has been accumulated on the test element. However, for measurement of thicknesses in an electroplating operation, such electrical resistance sensors must be connected to the high current plating power source, just as is the object to be plated. In such a situation, as the object is plated, the test element is plated, and its resistance decreases. Measurement of the decreasing resistance of the plated test element during progress of the electrolytic plating theoretically would provide a measurement of the plating thickness of the object. However, use of such electrical resistance measurement probes for an electrolytic plating bath has not been possible in the past. This is because the electrical current flowing in the electrolytic plating liquid is very large and causes so much interference with the measuring circuitry that measurement of resistance is not possible.
In a co-pending application, Ser. No. 821,013, filed Jan. 21, 1986, by Moore, Silverman and Bredow, and assigned to the assignee of the present application, there is described an electrical resistance sensor for monitoring electrolytic plating which avoids the problem of measurement interference caused by the high plating current. In the arrangement described in the co-pending application the test element is modified to provide a center tap which is connected to the plating power supply or a point of fixed potential. The central location of the tap causes plating currents flowing between the test element and the bath to flow in equal and oppositely directed components through the two halves of the test element on either side of the center tap, and thus the heavy interfering plating current effects are canceled from the resistance measurement.